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The Earth’s gravitational field’s potential energy, heat energy (such as geothermal), chemical energy, electric potential, and nuclear energy (from nuclear fission or nuclear fusion) are some of the available energy sources. Heat engines are especially important because many of these processes produce heat as an intermediate form of energy.

Atmospheric convection cells, for example, are one natural process that transform environmental heat into motion (such as rising air currents). Mechanical energy is particularly important in transportation, but it is also used in cutting, grinding, crushing, and mixing in many industrial processes.

Through a variety of thermodynamic processes, mechanical heat engines turn heat into work. Perhaps the most well-known example of a mechanical heat engine is the internal combustion engine, in which heat from the combustion of a fuel causes rapid pressurization of the gaseous combustion products in the combustion chamber, causing them to expand and drive a piston, which turns a crankshaft. In accordance with Newton’s third law of motion, a reaction engine, such as a jet engine, produces thrust by exchanging reaction mass, in contrast to internal combustion engines.

Pneumatic motors make use of compressed air, clockwork motors in wind-up toys use elastic energy, and electric motors convert electrical energy into mechanical motion, in addition to heat engines. A chemical engine, but not a heat engine, molecular motors, like myosins in muscles, use chemical energy in biological systems to generate forces and, ultimately, motion.